Modern car industry is increasingly seeking low-noise vehicles, i.e. vehicles preventing noise to be emitted to the interior or the exterior. For reducing the interior noise caused by the rolling wheels, the wheel housing outer liners and the aerodynamic under floor shields are provided with sound proofing materials. For reducing the exterior and the interior noise caused by the engine, the size and the number of the apertures of the engine compartment are minimized and the amount of absorbing material around the engine is maximized. Because of the presence of dirt, sand, water, snow, salt, ice and liquids like oils, coolant, battery fluid, brake fluid, fuels, washer liquid with anti-freeze and many other contaminating materials, the sound proofing materials located outside the passenger and trunk compartments need to be protected.
As disclosed in EP-0,229,977 it is known to use as a first measurement of protection a hydrophobic and oleo-phobic porous face fabric or a thin foil. This type of protection is sufficient for parts located near to the belt line of the vehicle, typically near to the engine top or the hood, like hood absorbers or water box absorbers, but generally it is not suitable for absorbers used near to the ground, like the under engine shield absorbers, as disclosed in FR-2,387,822, for which imperviousness and a certain ruggedness is required. It is evident for the man skilled in the art of acoustics that such qualities reduce the absorption properties of the absorber.
WO2005/007458 discloses an acoustically effective wheel housing which consists of at least three open pored layers having different air permeability. Unfortunately this kind of absorber sooner or later attracts external agents (dirt or salt in winter, humidity) which destroy the acoustic performance and increase the weight of the whole part.
It is also known to use wear resistant closed celled absorbers, as disclosed in WO02/066312, for manufacturing light-weight and noise reducing wheel housing elements, instead of open celled or fibrous absorbers sealed in a bag. Known closed celled polypropylene foam absorbers, in particular formed as a chamber absorber, as described in WO99/44816 or JP-58177781, are lightweight and possibly low cost but their absorption properties are poor (absorption is rarely above a=0.6), especially if the size of the chambers (height, width or length) is below 20 mm and the chamber walls are becoming too stiff, and little energy being dissipated inside the wall material itself.
Sealed absorbers generally are made of a non-woven felt die cut blank, generally cotton felt with phenolic resin, or alternatively of a slab of open cell PUR foam placed in a bag made of two foils welded together. The absorber is affixed to the under floor shield by fasteners, double sided tape or more generally by high frequency welding points. The acoustic absorption behaviour of a sealed single layer open celled absorber is generally spiky in the frequency domain (absorption peak has a narrow frequency bandwidth) and the maximum of the absorption value a is generally smaller than 0.8.
As known from PCT/CH2004/000572 it is acoustically advantageous to replace the known sealed single layer absorber by a two felt layer structure sealed in a bag. The sealed absorber described in above application comprises a top felt layer which exhibits an air flow resistance of between 300 to 1200 Ns/m3 (air flow resistance may be unambiguously determined using measuring methods as stated in ISO 9053 norm) and a bottom felt layer which has a thickness of between 1 to 20 mm, typically 5 to 10 mm. Both felt layers have an area weight of between 300 to 1200 g/m2. By using this layout a maximum absorption of a=1 could be reached, but more importantly this absorption shows a much wider peak frequency bandwidth. Unfortunately the area weight of the obtained laminate is typically around 1000 g/m2, which is rather heavy especially compared with the above mentioned chamber absorber.
In view of WO01/89883 a further critical parameter is the thickness of the absorber in a bag. A reduction of the thickness of the absorber shifts its maximum absorption frequency fo to a higher frequency region. If one tries to reduce the thickness of the absorber one takes the risk that the absorption spectrum of the absorber does not correspond anymore to the averaged noise spectrum of the engine or rolling noise, which the absorber was supposed to dissipate. For example a 10 mm thick felt absorber with a 60 g/m2 foil has its maximum absorption peak in the region of about 2500 Hz, while a rolling noise peak is generally generated at around 1000 Hz.
Unfortunately all of the known products for reducing the exterior and the interior noise caused by the engine or the rolling wheels of an automotive vehicle show well recognized limitations and shortcomings. The known noise reducing wheel housing outer liners or under floor shields provide either a poor degree of sound absorption, tend to wear off and degrade over time, are heavy and space consuming, or absorb frequencies in a small bandwidth only. Most of them are difficult and expensive to manufacture.